The id column stores activity IDs and the effort column stores effort descriptions.
The start_index and end_index columns store each effort’s start and end indices in the corresponding activity’s stream.
These indices can be used to compute the distance travelled, time elapsed, and mean pace achieved during each effort:

The values in the distance column of best_effort_paces differ slightly from the distances described in its effort column because there is not always a sequence of consecutive stream observations that cover such distances exactly.
However, I expect that, on average, the multiplicative error in distance equals the multiplicative error in time, so that the mean_pace column provides unbiased estimates of my true mean paces.
The chart below uses these estimates to track my personal record 5k, 10k, and half-marathon mean paces since joining Strava in late 2017.

I improved my 5k and 10k personal records significantly while training for, and competing in, the Wellington Waterfront 5k series during the 2018/19 and 2019/20 summers.
My mean pace during Round the Bays in February 2020 improved on my previous half-marathon record by 17 seconds per kilometre, but the fastest of the half-marathons I have run since improved on my Round the Bays pace by one second per kilometre only.